This invention relates to electrochemical sensing devices utilizing membranes to separate the electrolyte needed by the device from the medium containing the analyte of interest. More specifically, it relates to: (1) gas sensing devices wherein the membrane is utilized to separate the electrolyte from a liquid containing the gas being analyzed, (2) gas sensing devices wherein the membrane is utilized to separate the electrolyte from a gas phase, and (3) ion sensing devices wherein a selectively permeable membrane separates the liquid electrolyte utilized by the device from the analysis solution, which contains an analyte able to diffuse freely through the membrane.
Included in gas sensing devices wherein the membrane is utilized to separate the electrolyte from a liquid containing the gas being analyzed are the sensors utilized for transcutaneous blood gas monitoring, for clinical laboratory analysis of blood gases, and for laboratory and field measurement of gases such as ammonia, carbon dioxide, oxygen and chlorine. Also included in this category are intravascular (catheter) electrodes with proximal or remote reference electrodes for measuring oxygen or carbon dioxide.
Gas sensing devices wherein the membrane is utilized to separate the electrolyte from a gas phase include electrochemical sensor utilized for measuring gases such as oxygen, carbon dioxide, chlorine, or ammonia in flowing gas streams. Such devices are currently utilized for verifying the oxygen content of gas mixtures used for respiratory therapy.
Ion sensing devices wherein a selectively permeable membrane separates the liquid electrolyte utilized by the device from the analysis solution which contains an analyte able to diffuse freely through the membrane, include, specifically, intravascular electrodes for measuring blood pH, sodium, potassium and glucose.
The invention consists of a method and apparatus to simplify the process of changing the membrane and the electrolyte needed by the electrochemical sensing device. While it is believed that the major value will be with gas sensing devices, the invention can be used with any of the electrochemical devices described above which use replaceable membranes.
Basic to the operation of electrochemical sensors is the presence of an electrolyte, an ionically conducting medium, contacting both the anode and the cathode. In voltammetric oxygen sensors of the Clark type or potentiometric carbon dioxide sensors of the Stow-Severinghaus type or other gas sensors of the types disclosed by Ross and Riseman, this electrolyte is an aqueous solution, sometimes modified by other water-compatible solvents such as ethylene glycol or propylene glycol or glycerol. At times, these other solvents may make up the bulk of the electrolyte solution and the water content may vary from traces to only a few percent.
The membrane in these devices may serve several functions. It can be utilized to prevent evaporation of the electrolyte solvent or to prevent fouling of the electrodes. It can prevent contamination of the electrolyte solution or changes in the solution concentration. It can be selectively permeable, allowing only gases to enter for analysis, or it may allow ions and not proteins to reach the sensing electrodes. It can be a diffusion barrier and provide most of the concentration gradient between the medium being analyzed and the electrode where the analyte is being consumed. It can control the thickness of the electrolyte layer and, under some conditions, control the sensitivity of microelectrodes.
The output stability, i.e., the ability to maintain a reproducible output signal for periods ranging from hours to days when the sensor is exposed to a reproducible gas composition, in both voltammetric and potentiometric sensors, is dependent upon the maintenance of a constant composition in this electrolyte, although the reason for this is different in the two types of sensors.
In voltammetric sensors, e.g., oxygen, stability is dependent upon maintaining the constancy of the diffusional pathways to the cathode. This means that both the geometry of the diffusion layer and any of its properties which affect transport, such as solubility and diffusion coefficient, should remain constant; the response of an oxygen sensor is relatively insensitive to the absolute concentration of any of the ions. With potentiometric sensors, on the other hand, the concentration of one or more of the ions is critically important; for carbon dioxide sensors, a common stability specification requires that concentration changes be limited to less than one percent per hour.
While many factors may have a significant effect upon electrolyte properties, an important one is the diffusion of water vapor through the membrane. This directly affects the concentration of electrolytes, so affecting the readings of potentiometric gas sensing electrodes, and for electrolytes with a low water content, changes may have a strong effect upon the diffusional properties of the electrolyte and so affect voltammetric sensors. Although membranes are typically made from rather hydrophobic polymers, good transient response and a number of engineering considerations have limited the number of satisfactory materials to a small number with intermediate transport properties for oxygen and carbon dioxide and concomitantly, for water vapor.
The need for good transient behavior limits the thickness of the electrolyte layer between the membrane and the sensing electrode. For many electrode configurations, such as transcutaneous devices and intravascular devices, it has heretofore proven impossible to provide a large electrolyte reservoir, and, therefore, the electrolyte has had to be replenished at regular intervals.
Historically, membrane-based electrochemical sensing devices have been prepared for use by placing a small volume of electrolyte so that it is contained between the surface of the structure which contains the electrodes and the membrane which separates the sensor and its electrolyte from the medium being measured. The membrane may be fixed in position by a variety of means, ranging from a rubber band or O-ring to a structure which holds the membrane and which can be fastened to the sensor by screw threads, an interference fit, or an over center, snap-like device. Mechanical aids may be used to facilitate assembly.
With electrolytes used heretofore, the user should add a small volume, usually less than a milliliter and often a drop or less, of the electrolyte to the electrode face or the membrane surface, and this usually requires a fresh membrane because the previous application stretched the membrane enough so that reapplication produces a loose fit. There are several other reasons why membranes have to be replaced at regular intervals--mechanical damage, membrane fouling, the need to polish the electrode surface (in the case of oxygen sensors), and electrolyte evaporation. While changing a membrane is not a difficult operation, it is not only an inconvenience but a continuing potential source of operational errors, and in the clinical environment, especially, any simplification has value.